Summary: Finally, arc diff works again! This has been sitting in my repo for a while. I would like some comments on my BlobStore benchmark. We don't have to check this in. Also, I don't do any fsync in the BlobStore, so this is all extremely fast. I'm not sure what durability guarantees we need from the BlobStore. Test Plan: Nope Reviewers: dhruba, haobo, kailiu, emayanke Reviewed By: dhruba CC: leveldb Differential Revision: https://reviews.facebook.net/D13527